Area of the Art
The present invention relates to a class of novel molecules with Michael acceptors. These molecules are based from a bis-benzylidine piperidone backbone and can be used as therapeutic agents against various types of cancers. Specifically, these molecules work as proteasome inhibitors and bind to the RPN13 subunit of the 19S regulatory particle.
Description of the Background Art
Protein degradation is exquisitely regulated within the cell to maintain protein homeostasis and eliminate misfolded or damaged proteins.32 Targeted degradation of regulatory proteins by the ubiquitin-proteasome system (UPS) is central to many signaling cascades including those that govern cell proliferation and is exploited by many infectious agents.12 The degradation of a target protein is signaled by repeated covalent linkage of ubiquitin mediated by E3 ubiquitin ligases, of which hundreds have been described. Upon the attachment to the target of extended chains of ubiquitin, each conjugated via lysine 48, the poly-ubiquitinated proteins are recognized by two proteasome subunits, RPN10 and RPN13 within the 19S regulatory particle (RP).19,30,31 The 19S RP recycles the ubiquitin by its removal from the target protein, and unfolds the target protein while passing it to the 20S core particle of the proteasome for degradation. RPN13 binds to both UCH37, enhancing its deubiquitinase activity28, and to RPN2 that modulates the translocation and subsequent degradation of substrates by the 20S.11 The 20S core particle contains three catalytic subunits, β1, β2 and β5, with caspase, trypsin, and chymotrypsin-like activities respectively.32 Degradation occurs progressively via nucleophilic attack of the substrate amide bond by a Threonine within the β-subunit active site. The inhibitors bortezomib and carfilzomib principally inhibit chymotrypsin-like proteolysis.7 
The increased reliance upon proteasomal function in cancer cells can provide a therapeutic window.1,7 Bortezomib was approved for the treatment of relapsed multiple myeloma (MM) and mantle cell lymphoma7, and Carfilzomib was recently approved for patients with MM progression while on or after treatment with bortezomib and an immunomodulatory agent. The efficacy of these proteasome inhibitors has been attributed to the activation of the unfolded protein response (UPR) and endoplasmic reticulum stress due to toxic accumulation of protein aggregates, inhibition of NF-κB and TNFα signaling, increases in reactive oxygen species (ROS) and stabilization of tumor suppressors such as p53.10 In human papillomavirus (HPV)-related cancers the E6 viral oncoprotein drives transformation by co-opting the cellular E3 ubiquitin ligase E6AP to polyubiquitinate target E6-binding proteins, notably tumor suppressors such as p53 and PDZ-family members including DLG-1, and trigger their rapid degradation.18,25,26 Preclinical findings suggest that HPV-transformed cells are preferentially sensitive to bortezomib as it recovers their levels of E6-targeted tumor suppressor proteins, and triggers apoptosis.23,32 
Unfortunately, bortezomib induces thrombocytopenia and neuropathy (associated with off-target activity,4 and the emergence of disease resistance remains a clinically significant problem.29 Carfilzomib has similar issues. New orally delivered drugs targeting distinct activities of the proteasome are needed to increase dosing flexibility, overcome resistance and reduce side effects.9 Here we describe a new compound, RA190, which is orally bioavailable, which inhibits proteasomal degradation by binding to a novel proteasome target, the ubiquitin receptor RPN13, which and shows promising activity against bortezomib-resistant MM, ovarian and HPV-associated cancers.